Molten metals processed in atmospheric air tend to oxidize and lose alloying additions, form slag causing difficulties in handling and wear of refractory material causing formation of non-metallic inclusions, absorb unwanted nitrogen and hydrogen from the air, resulting in poor metal quality and/or toxic fumes. In the past in order to minimize these problems, various protective coverings were used on a bath of molten metal exposed to the atmosphere. Examples of prior art techniques were the use of graphite or charcoal covers, liquid fluxing salts, synthetic slags, protective gaseous atmospheres or enclosing the vessel in a vacuum enclosure.
In the past, liquified cryogenic gases (e.g., nitrogen and argon) were successfully tried as a means for protecting molten metal surfaces. Use of direct application of liquified cryogenic gases to the molten metal surface has been limited because of lack of properly designed cryogenic sprayers that would assure uniform dispersion of the liquid cryogen over a large molten metal surface area without entraining excessive amounts of ambient atmosphere or excessive boil-off losses of cryogenic liquid. The prior art systems required an overly complex and/or manifolded piping, increased cost if liquified argon was used to blanket melts because of the composition of the reel t. The danger of a cryogenic liquid explosion is present if a concentrated and poorly dispersed stream of cryogen was trapped between the molten metal surface and a crust or layer of oxides or slag located on the surface of the molten metal.
The importance of dispersing of the cryogenic liquid in a proper fashion was largely unrecognized in the art. Foulard, et al. (U.S. Pat. No. 4,518,421) disclosed a process of evaporation-condensation refining of molten metals in a semi-closed container using a relatively straight tube to deliver cryogenic liquid to the molten metal surface.
Gilbert, et al. (U.S. Pat. No. 4,178,980) disclosed an annular phase separator to protect the stream of molten metal cast into a mold. The Patentees discharged the cryogen through inclined angular nozzles in the bottom of the annular separator thus minimizing air aspiration.
Devalois, et al. in U.S. Pat. No. 4,460,409 disclosed using a partly immersed converging cylindrical tube to confine the molten metal surface area being blanketed with the liquefied cryogen which is discharged through a narrow ended tube.
Anderson, et al. (U.S. Pat. No. 4,990,183) proposed blanketing an uncovered molten metal surface with liquid argon discharged either by a tube or a porous diffuser-separator under a closed lid covering ladle, laddles or laddle furnaces.
Borasci, et al. (U.S. Pat. No. 4,915,362) disclosed a carbon dioxide snow nozzle used to discharge massive amounts of this relatively inexpensive, but not really inert, solidified gas in order to compensate for the operating costs and the surrounding air entrained over the covered area by use of a high-velocity carbon dioxide jet.
The prior art shows the placement of cryogenic liquid near the covered molten metal surface limits entrained air and gas consumption/cost minimization were more or less successfully attempted with complex and difficult to implement geometrical arrangements around the cryogenic discharging devices or by compromising efficiency of uniform blanketing with cheaper reactive cryogenic gases or undeveloped cryogenic spray-separators.